1. Field of the Invention
The present invention relates to a semiconductor device which operates similarly to a semiconductor device having a MOS gate structure such as a MOSFET or an IGBT, and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, semiconductor devices having MOS gate structures have frequently been used as a power device and an RF device. FIG. 37 is a cross sectional view showing a power MOSFET as an example of a conventional power device.
In FIG. 37, reference numeral 92 denotes an n-drift layer. A p-well layer 93 is selectively formed in the surface of the n-drift layer 92, and a low-resistance n-source layer 94 is selectively formed in the surface of the p-well layer 93.
A gate electrode 97 is arranged on the p-well layer 93 between the n-drift layer 92 and the n-source layer 94 through a gate insulating film 96. A source electrode 95 is arranged to contact both the p-well layer 93 and the n-source layer 94. A drain electrode 98 is arranged below the n-drift layer 92 through a low-resistance n-semiconductor layer 91.
In a power MOSFET of this type, the semiconductor layers such as the p-well layer 93 and the n-source layer 94 are formed by diffusing impurities. For example, the p-well layer 93 is formed by diffusing a p-impurity such as boron, and the n-source layer 94 is formed by diffusing an n-impurity such as arsenic.
For this reason, the power MOSFET has the following problems. More specifically, since layer formation by impurity diffusion takes a long period of time, formation of the semiconductor layers such as the p-well layer 93 and the n-source layer 94 takes a long period of time to prolong the manufacturing time of a device. In particular, it is impossible to manufacture a device when a semiconductor made of SiC, CdS, diamond, or the like, in which impurity diffusion is difficult and an impurity ionization rate is low, is used as a bulk material.
Further, in a semiconductor device having a MOS gate structure such as a power MOSFET, a current is caused to flow through a channel generated by control of the gate electrode 97, undesirably generating a channel resistance. Such a channel resistance causes an increase in ON state voltage, so that it is difficult to improve the ON state characteristics. In particular, the channel resistance is increased in SiC and a semiconductor device of this type is hardly realized.
As described above, the conventional power MOSFET has the problem of an increase in manufacturing time of the device because the device is formed by diffusing impurities. In addition, it has the problem of an increase in ON state voltage owing to the presence of the channel resistance.